There are a couple of couples (hah) that I mainly see at dinner every few weeks, and we've just ended up with an informal system such that we take turns paying the whole bill, with the person/couple that picked the restaurant being the ones to pick up the tab. So we also take turns picking restaurants. It seems to work out pretty well! If it's my turn to pick and I'm feeling especially poor, I'll just pick somewhere cheaper or offer to cook dinner at my place. But if it's my turn to pick and one of them just had a birthday or something, I'll pick something swanky.